IU 


k,  JM1  OF  WSTRY,  UBBARY 

TECHNICAL     NOTE     number      172 


FOREST    PRODUCTS    LABORATORY     -    U.    S.    FOREST    SERVICE        MADISON.    WISCONSIN 


HOW  TO  OBTAIN  RIGIDITY  IN  CRATE  CONSTRUCTION 


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One  of  the  features  of  a  good  crate  is  rigidity 
or  ability  to  resist  weaving  and  skewing  during  trans- 
portation. No  method  of  joining  the  corner  members  of 
a  crate,  not  even  the  3-way  corner  construction,  is 
sufficient  alone  to  give  rigidity  to  a  crate.  Some 
kind  of  bracing  across  the  faces  is  usually  necessary. 

Figure  1  shows  a  kind  of  bracing  found  in  many 
crates  which  are  sent  to  the  U.  S  Forest  Products  Labo- 
ratory, Madison,  Wisconsin,  for  testing.  Partly  because 
of  the  amount  of  material  used,  this  construction  ap- 
pears to  be  very  strong.  Laboratory  tests  h^y^&howrr] 
however,  that  crates  so  braced  are  weak  in  the  diagonal 
direction  of  the  faces,  and  are'  therefore  apt  to  weave 
and  skew  during  transportation.'      m 


FIG.  1 


FIG.  2 


FIG.  3 


Diagonal  braces  on  six  sides  as  shown  in  Figure 
2  have  been  found  to  give  a  crate  maximum  rigidity  for 
a  minimum  amount  of  lumber.  Crates  so  braced  withstood 
with  considerably  less  distortion  twice  as  great  a 
diagonal  compressive  force  in  actual  tests  as  crates 
braced  as  shown  in  Figure  i. 


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A  combination  of  diagonal  and  parallel  slat  "brac- 
ing, as  shown  in  Figure  3,  makes  the  crate  more  rigid 
than  parallel  bracing  alone  but  not  so  rigid  in  all  di- 
rections as  cross  bracing  on  the  six  sides.  It  maybe 
found  an  advantageous  construction  in  packing  contents 
which  need  protection  on  the  sides  and  are  rigid  enough 
themselves  to  withstand  stresses  in  the  direction  in 
which  the  crate  is  weak. 

Solid  sheathing  on  all  the  faces  does  not  make 
a  crate  so  rigid  as  diagonal  bracing,  except  perhaps 
sheathing  which  is  made  of  wide  boards  with  tighter 
joints  than  can  usually  be  obtained.  The  crate  with 
ordinary  sheathing  might  withstand  as  great  a  load, 
but  the  distortion  caused  by  that  load  would  be  greater 
than  in  a  crate  with  diagonal  braces,  and  would  ordi- 
narily be  great  enough  to  allow  damage  to  the  contents. 


